King's were awarded a penalty in the very first minute allowing Martyn Bowles to kick over and get the scoring underway.
RGS responded with some positive play but Joe Taylor stormed through their defence and had to be thwarted by Jonny Hughes at full-back to deny King's a first try.
Bowles had another kicking opportunity but missed, and then Will Dovey thought he had scored a try, only for the referee to bring play back for a RGS penalty.
And King's were punished when Billy Grant raced down the right wing to put RGS 5-3 ahead.
They almost found themselves further behind when Jack Tromans darted towards the tryline but a knock-on ended the move.
Hughes impressed for RGS again with a rapid 50-metre run, shaking off tackles before eventually being taken down in his opponents' half.
RGS came agonisingly close to extending their lead when a superb break saw captain Bee fall just a few metres short of the tryline.
Jake Saunders was a threat for RGS but Ryan Kerley snatched a try in the final minute of the first-half to give King's an 8-5 lead at the interval.
The second period was a hard-fought battle and it was Hughes again threatening King's but he was taken down just short of the whitewash.
RGS exerted more pressure and it paid off when Bee powered over the tryline and Tromans converted to put his side 12-8 ahead.
Ryan Kerley had a penalty opportunity to reduce the deficit but his kick didn't reach the upright and RGS sealed victory when Tromans punted over a late kick.